The research and development leading to the subject matter disclosed herein was not federally sponsored.
This invention relates to reinforcing materials for concrete and concrete structures so reinforced.
Concrete is one of the most common building materials. It is used in a wide variety of structures such as bridges, walls, floors, building supports, roadways, and runways among many others.
For several reasons, concrete structures are usually made with some sort of reinforcement. Concrete is often prone to cracking as the structure is weathered or subjected to bending loads and impact. This is mainly due to the poor tensile properties of the concrete. Reinforcing materials are commonly used to improve the tensile properties of concrete structures. In addition, concrete is applied wet and in some instances must hold its position shape (against, e.g. the force of gravity) until it hardens. Sometimes reinforcing materials are added to the concrete to help hold the mass together and in position until it sets.
Concrete reinforcements come in several types. Reinforcing bars are common. These are typically steel but are sometimes a thermoset/fiber composite. A second type of reinforcement is an overwrap. The overwrap is commonly a thermoset/fiber composite that is applied to the outside of a structure. Overwraps of this sort are often used to shore up a cracked or damaged structure, or to strengthen structures so they become more resistant to natural phenomena such as hurricanes, tornadoes and earthquakes. Overwraps are not limited to concrete structuresxe2x80x94they can be applied to structures of many types of construction, such as brick, stone, and frame constructions.
A third type of concrete reinforcement is fibers that are embedded in the concrete. The fibers used in this application are usually steel or polypropylene. These fibers are short, commonly of the order of 12-50 mm in length, and typically have a diameter of around 0.1-1 mm. The fibers are mixed into the wet concrete. When the concrete is poured, the fibers become randomly oriented in the concrete, forming a xe2x80x9cfuzzyxe2x80x9d matrix that helps prevent cracking or crack propagation. This matrix also helps hold the wet concrete together until it can harden.
The common steel and polypropylene fibers each have significant limitations. Steel fibers are very strong and stiff, but they are difficult to handle and apply. They are prone to corrosion when exposed to water and salts. Polypropylene fibers do not corrode, but are undesirably ductile and not as strong as desired. Further, with all fibers but especially strong stiff fibers such as steel, it is relatively difficult to generate the full strength of the fibers since they do not bond adequately to the concrete so that when a load is applied, they tend to pull out below their ultimate failure strength.
Glass fibers would have an excellent combination of stiffness, strength and resistance to corrosion, but they are too brittle for this application. The process of mixing glass fibers into the concrete and pouring the concrete breaks the fibers up into short lengths that do not provide much reinforcement. In addition, glass fibers are not chemically stable in the alkaline environment of concrete.
In order to overcome the deficiencies of glass fibers, it has been attempted to provide them with a polymeric coating. The polymeric coating would be expected to reduce the friability of the glass fibers as well as protect them from the alkalinity of the concrete. However, it is difficult and expensive to provide glass fibers with a suitably thin coating that also completely covers the fibers.
Thus, it would be desirable to provide an improved method by which reinforcement can be provided to concrete, which provides high strength and stiffness combined with ease of handling, no corrosion and excellent mechanical and/or chemical bonding into the concrete.
In one aspect, this invention is a composite adapted for use in a concrete structure, the composite comprising a plurality of longitudinally oriented fibers embedded in a matrix of a depolymerizable and repolymerizable thermoplastic resin, said composite having a longest cross-sectional dimension of not more than about 5 mm and an aspect ratio of at least 10.
In a second aspect, this invention is a concrete structure reinforced with up to 10 volume-percent of a small cross-section composite, said small cross-section composite comprising a plurality of longitudinally oriented reinforcing fibers embedded in a matrix of a depolymerizable and repolymerizable thermoplastic resin, said small cross-section composite having a longest cross-sectional dimension of not more than about 5 mm and an aspect ratio of at least 10.
In a third aspect, this invention is a method of making a reinforced concrete structure, comprising the steps of (a) forming a wet concrete mix containing a mortar or cement, a particulate filler and up to 10 volume-percent of a small cross-section composite, said small cross-section composite comprising a plurality of longitudinally oriented fibers embedded in a matrix of a depolymerizable and repolymerizable thermoplastic resin, said small cross-section composite having a longest cross-sectional dimension of not more than about 5 mm and an aspect ratio of at least 10, (b) shaping the concrete and (c) permitting the concrete to cure.